In recent years, an electro photographic digital color copying machine, an ink-jet or thermal transfer color printer, etc. (image forming device) have been widely spread due to rapidly developed digitalization of office automation equipment and increasing demand for color image output.
In these image forming device, image information inputted from an image input device such as digital camera and scanner, or image information created on a computer are outputted through an image output device.
Such image forming device needs to output images with constantly stable color reproduction, corresponding to image information inputted. In the realization of such images, color correction in the digital image processing technology plays an important role. Note that color correction processing herein refers to all of the following operations: correction of color signals that would be inappropriate signals when outputted directly; conversion of color signals for a change in color itself (color conversion); and transformation of color signal's coordinates without any change in color itself.
As to a method for color correction, many proposals have been conventionally made including color coordinate transformation for converting input image data into uniform color space data. Such a method includes a Look-Up Table (hereinafter referred to as LUT) method such as direct conversion and three-dimensional interpolation, and masking method, which are described in “Color Science Handbook New Edition” (Edited by The Color Science Association of Japan; published by Tokyo University Press p. 1137-1149) and Journal of Image Society of Japan (No. 4 (1998), Vol.37, p.555-559).
Such color correction is carried out in an image processing device of the image forming device. Examples of color correction by the image processing device include color correction of input from an image input device such as scanner (pre-process of color correction), and color conversion which is proper processing to provide image formation of image data inputted from the image input device onto a recording material such as paper by electro photographic or ink-jet image output device (post-process of color correction).
The color correction (pre-process and post-process of color correction) will be explained below.
First, the pre-process of color correction will be explained.
Usually, image data inputted from an image input device such as scanner are data for a color space of the image input device.
Therefore, the image processing device must provide a removal of various distortions caused by an illumination system, imaging system, and capturing system, for the image data inputted from the image input device (pre-color correction).
For example, 3×3 matrix coefficient obtained by the least-square method is used to remove the distortions in such a pre-process of color correction.
Next, the post-process of color correction will be explained.
The post-process of color correction is a conversion of RGB signals into CMY(K) signals after removing turbidity based on spectral characteristic of CMY (C: Cyan, M: Magenta, and Y: Yellow) colors, including unnecessary absorption elements when the RGB signals are expressed using CMY(K) colors, in order to reproduce the original colors faithfully.
For example, Japanese Unexamined Patent Publication No. 220660/1985 (Tokukaisho 60-220660; published on Nov. 5, 1985; conventional art (A)) discloses a color correction process to minimize a color difference between an original image and its reproduced image, as a post-process of color correction.
In this color correcting method, firstly, color-separated signals R0, G0, and B0 are identified for each pixel, which area they belong to among predetermined plural areas in a color separation signal space. Then, color correcting coefficient matrices are obtained in accordance with the result of the identification, and correcting color-separated signals R1, G1, and B1 are outputted using color correcting coefficient matrices.
This allows color correction corresponding to the area in the color-separated signal space, i.e. the value of supplied color-separated signals R0, G0, and B0. Consequently, a color difference between an original image and its reproduced image can be minimized, thereby improving an overall color correction accuracy.
Note that, the color correcting coefficient matrices are predetermined in accordance with the area of the color-separated signal space, and prepared in plural number.
Further, Japanese Patent No. 3133409 (conventional art (B); granted on Nov. 24, 2000; Japanese Unexamined Patent Publication No. 48888/1993 (Tokukaihei 5-48888; published on Feb. 26, 1993) discloses a digital color copying device which provides adjustment of the same fine hue adjacently positioned, as a post-process of color correction.
The digital color copying device is provided with a plurality of color conversion tables in accordance with the hues of an original as well as selects a proper color conversion table in accordance with the judging result of the hue of the original. This makes it possible to improve the reproducibility of a color original in which the same fine hue is adjacently positioned.
Japanese Unexamined Patent Publication No. 193477/1997 (Tokukaihei 9-193477; published on Jul. 29, 1997; conventional art (C)) discloses a printing device which provides color correction to each object of an printed image, as a post-stage correction process.
In the printing device, the type of each object (graphics, text, photograph) is judged with reference to printed data described in page description language (PDL), and color correction is carried out using a color correction table corresponding to the type of judged object. This makes it possible to provide color correction for each object, thereby obtaining an appropriate printed image.
Incidentally, to the image processing device which provides such color correction, image data are inputted from the image input device, as described above. Namely, the image input device scans images from an original and outputs them as image data to the image processing device.
Input characteristics of this image input device does not match human visual characteristics. Therefore, depending on the type of image (original), the image input device might scan an image with colors different from those by human visual perception. For this reason, some type of image needs correction of supplied signals in the image processing device.
However, neither the conventional art (A) nor (B) provides color correction (pre-process of color correction) in accordance with the types of image (e.g. text, screen dots, photograph, etc.) in the relation between the image input device and image processing device.
This cannot provide color correction in accordance with the types of image different in color correction accuracy.
Also, the conventional art (C) needs a plurality of color correction tables in accordance with the types of image to perform color correction in accordance with the types of image. This causes complication of its constitution and a necessity of increasing a storage capacity. This results in a higher cost for manufacturing the image processing device.
Further, it takes a lot of trouble to create such a color correction table. Accordingly, if a plurality of color correction tables is used to provide color correction depending on plural types of image, it is necessary to increase a step of creating the color correction tables.